The CECILIA2050 project has set out to identify an “optimal” mix of climate policy instruments to achieve Europe’s climate targets for 2050. In order to assess different policy instruments, CECILIA2050 will adopt a broad notion of optimality, which does not only analyse what looks best in theory but also what is the most expedient way forward under real-life constraints. Drawing on a survey of the relevant literature, this paper presents three main criteria that guide the analysis of policy instruments will thus inform the subsequent work in the CECILIA2050 project. These criteria are: policy effectiveness (is the policy contributing to a desired outcome), policy cost-effectiveness (distinguishing between static and dynamic efficiency), and policy feasibility (distinguishing between political, legal and administrative feasibility). For each criterion, the paper discusses how policies can be assessed in practice and how the criteria could be combined in an overall assessment of optimality. The paper also discusses the appropriate scope of analysis, arguing that an assessment of optimality should look beyond single instruments and rather assess the performance of the climate policy instrument mix as a whole. This includes the interactions between policy instruments and between climate instruments and adjacent policy areas, such as energy or transport policy.